The present invention relates to intrinsically safe logic AND circuits, i.e. circuits that are not liable to a failure leading to a logic level 1 at the output while one of the inputs is at a logic level 0 ( such a level 0 being indicated by an input voltage below a given threshold).
A particularly important, but not exclusive application of the invention lies in railway installations that may be designed so that the presence of such a zero level corresponds to a restriction of vehicle traffic over guiding tracks.
AND-circuits are already known that comprise, in cascade, a threshold oscillator that delivers an output signal only if the voltage on a power supply input (constitung a first input of the AND circuit) exceeds a determined threshold, and a peak-limiting amplifier that cannot provide an output signal exceeding a given value unless the voltage on its own power supply input (constituting the second input of the AND circuit) exceeds another threshold, said amplifier being designed so as to be unable to oscillate spontaneously.
The circuit is often completed with a rectifier for transforming the AC output signal representative of level 1 at the output from the implementing amplifier into a DC signal suitable for driving another AND circuit.
An impedance matching amplifier is generally also placed downstream from the peak-limiting amplifier and upstream from a possible rectifier.
It can be seen that such circuits operate with AC, thereby avoiding any risk of an output being directly fed through from an input as could happen in the event of DC operation. In general, coupling transformers are used between the various components since with appropriate installation such transformers completely block the passage of any DC signal. Such transformers may be associated with other kinds of impedance (capacitor and/or resistance) enabling a resonant circuit to be implemented. Any damping resistance must necessarily be connected in series so that in the event of a connection being broken there can be no increase in the output signal.
Existing logic AND circuits of the above-defined type suffer from various drawbacks and limitations. In particular, they are expensive since they require a large number of components and they require a metal screeming box for protection against electromagnetic interference. They are sensitive to alternating input signals in common mode that could induce an output signal in the absence of a ground node to which all of the components are connected. The screening box presents an antenna effect.